Artificial crystals get their own textbook laws

An alternative chemistry of DNA-bonded nanoparticles, rather than chemically bonded atoms, just got a boost. We now have rules for building crystals like this, which means they can be created on demand.

In nature, the atoms of different elements arrange themselves in very different ways to form crystals. How many partners each atom is bonded to, and the length of the bonds, depend on the sizes and properties of the atoms in question. So sodium chloride forms a “face-centred cubic” structure, in which each sodium atom is surrounded by six chlorines and vice versa; whereas in gallium arsenide, say, the atoms arrange themselves differently and each has only four nearest neighbours.

Chad Mirkin of Northwestern University in Evanston, Illinois, and colleagues wondered if they could wrest control from nature and create crystals where the bond lengths and number of bonds don’t depend on the size or composition of the component particles.

To do this the researchers coated their atom analogues – gold nanoparticles – with multiple DNA molecules. The DNA contained exposed, single-stranded sections that formed “sticky” regions on each particle which could bond to complementary sections on strands coating other gold particles.

Advertisement

It’s not the first time that researchers have used these building blocks to create artificial structures. In the past few years, structures have been build that resembled natural crystals, with the nanoparticles as “atoms” and the DNA linkers standing in for chemical bonds. A current limitation is that the identities of the particles being assembled often determine the structures that can be synthesised – so certain structures can only be built using certain nanoparticles and vice versa.

Now Mirkin and colleagues have worked out how to dictate the number of nanoparticles and the length of bonds for a system of particles of a given size and composition – and summarised their findings in six rules. For instance, the total size of the nanoparticle, including its DNA coating, determined what sort of crystal developed – and this size could be tailored either by using different lengths of DNA or different-sized nanoparticles.

Against nature

These rules could be used to design artificial crystals with totally novel properties, says Mirkin. This is “one of the most fundamental demonstrations of man over nature”, he adds.

For instance, he suggests using the rules to design materials that can absorb light of low energy and release it in the form of high-energy photons. This might drastically improve the efficiency of solar cells.

“There are extremely few successful examples of crystals with nanoparticles, and these were obtained after extremely difficult procedures and with little control over the final structure,” says Alex Travesset of Iowa State University in Ames. “This paper shows how DNA-programmed self-assembly provides a relatively simple route to solve this very fundamental technological problem.”